Unfortunately, GTK 4 has removed key themes. Eventually, Firefox, Chrom(e|ium) and all actual GTK applications will migrate to GTK 4, so this is not very future-proof.

I use a variant of the emacs key theme, so I'd be happy if there were a workaround...

It's unfortunate that the GTK devs feel they have to remove features which their users find useful to keep GTK maintainable. Keeping software complexity under control is a difficult task, and I can't begrudge them their choice.

You are a hero and have just cured me of a hundred daily paper cuts. My employer should give you a bonus.

It works in Chrome, but unfortunately not in the standalone Slack client. (Maybe an argument for switching to the web version?)

(Uh, does anyone know how I select all (previously C-a) or search (previously C-f) in Chrome now??)

EDIT: Looks like C-/ functions as select all, still looking for a find replacement.

EDIT #2: Oh, silly me, as long as I'm not focused on an input, C-f still works.